1. Field of the Invention
The present invention relates to an internal combustion engine equipped with an exhaust gas recirculation (EGR) system. More specifically, it relates to an intake manifold of such an internal combustion engine, for preventing the deposition of combustion products around the EGR gas outlet.
2. Description of the Related Art
In an ordinary internal combustion engine, unburnt hydrocarbons (abbreviated to "HC" hereinafter) leak through the clearances between the piston and the piston rings into the crankcase of the engine due to a so-called pumping action of the piston rings during reciprocating operation, of the piston. The discharge of HC (generally referred to as "the blow-by gas") into the atmosphere causes air pollution. Therefore, it is usual to feed the blow-by gas through the cylinder head into the intake manifold, where the blow-by gas mixes with the intake air or fuel-air mixture. Such a system for introducing the blow-by gas into the intake manifold is generally referred to as a positive crankcase ventilation system (abbreviated to "PCV system" hereinafter).
On the other hand, in some internal combustion engines, before discharging the exhaust gas into the atmosphere, a part of the exhaust gas (EGR gas) is circulated through the intake system, to reduce the concentration of nitrogen oxides (abbreviated to "NOx" hereinafter) in the discharged exhaust gas. In such an exhaust gas recirculation system, especially for a Diesel engine, EGR gas contains relatively large amount of combustion products, such as carbon particles. On the other hand, in a PCV system as mentioned above, oil vapor contained in the blow-by gas is brought from the crankcase through the cylinder head and head cover into the intake manifold in which the oil flows in a mist or film-like condition. Under these circumstances, the carbon particles contained in the EGR gas are trapped by the oil flowing from the PCV system into the intake manifold and adhered to the inner surface of the intake manifold. These particles, hereinafter referred to as "deposits," grow in the vicinity of the EGR pipe (the EGR gas inlet of the intake manifold), thereby these deposits may block the outlet of the EGR pipe. In an engine equipped with a turbocharger, some of the bearing lubrication oil in a turbin housing will leak to the intake manifold, so that the amount of oil flowing therein is increased, thereby the above-mentioned problems will readily arise.
Techniques have been proposed to reduce the collection of deposits around the outlet of the EGR pipe. Such known techniques are, for example; projecting the outlet of an EGR pipe into the interior of an intake manifold (Japanese Unexamined Utility Model Publication Nos. 56-88933 and 58-116748); disposing the outlet of an EGR pipe and the outlet of the pipe of a PCV system symmetrically with respect to the center axis of the throttle valve (Japanese Unexamined Patent Publication No. 58-65922); providing a blow-by gas guide for guiding the blow-by gas to a point downstream of the outlet of the EGR pipe (Japanese Unexamined Utility Model Publication No. 56-88934); or projecting a blow-by gas pipe of a PCV system into the interior of an intake manifold (Japanese Utility Model Publication No. 58-33713).